Phosphor Based Illumination System Having a Plurality of Light Guides and a Display Using Same

ABSTRACT

An illumination system including a light source, light guides coupled to the light source, each including an input surface and an output surface, emissive material positioned to receive light from at least one light guide, and a first interference reflector positioned between the emissive material and the output surfaces of the light guides is disclosed. The light source emits light having a first optical characteristic. The emissive material emits light having a second optical characteristic when illuminated with light having the first optical characteristic. The first interference reflector substantially transmits light having the first optical characteristic and substantially reflects light having the second optical characteristic.

RELATED PATENT APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/884,649, filed Jun. 30, 2004, now allowed, the disclosure of which isincorporated by reference in its entirety herein.

The following co-owned and copending U.S. patent applicationpublications are incorporated herein by reference: 2006/0002108(Ouderkirk et al.), 2006/0002101 (Wheatley et al.), 2006/0002678 (Weberet al.), 2006-0002141 (Ouderkirk et al.). The co-owned and copendingU.S. Pat. No. 7,182,498 (Schultz et al.) is incorporated herein byreference.

BACKGROUND

White light sources that utilize light emitting diodes (LEDs) in theirconstruction can have two basic configurations. In one, referred toherein as direct emissive LEDs, white light is generated by directemission of different colored LEDs. Examples include a combination of ared LED, a green LED, and a blue LED, and a combination of a blue LEDand a yellow LED. In another configuration, referred to herein asphosphor-converted LEDs (PCLEDs), a single LED generates light in anarrow range of wavelengths, which light impinges upon and excites aphosphor or other type of emissive material to produce light havingdifferent wavelengths than those generated by the LED. The phosphor caninclude a mixture or combination of distinct emissive materials, and thelight emitted by the phosphor can include broad or narrow emission linesdistributed over the visible wavelength range such that the emittedlight appears substantially white to the unaided human eye.

An example of a PCLED is a blue LED illuminating a phosphor thatconverts blue light to longer wavelengths. A portion of the blueexcitation light is not absorbed by the phosphor, and the residual blueexcitation light is combined with longer wavelengths emitted by thephosphor. Another example of a PCLED is an ultraviolet (UV) LEDilluminating a phosphor that absorbs and converts UV light either tored, green, and blue light, or a combination of blue and yellow light.

Another application of PCLEDs is to convert UV or blue light to greenlight. In general, green LEDs have a relatively low efficiency and canchange output wavelength during operation. In contrast to green LEDs,green PCLEDs, can have improved wavelength stability.

Advantages of white light PCLEDs over direct emission white LEDs includebetter color stability as a function of device aging and temperature,and better batch-to-batch and device-to-device coloruniformity/repeatability. However, PCLEDs can be less efficient thandirect emission LEDs, due in part to inefficiencies in the process oflight absorption and re-emission by the phosphor.

SUMMARY

The present disclosure provides illumination systems that utilizeemissive materials and interference reflectors for filtering components.In some embodiments, the interference reflectors of the presentdisclosure may include multilayer optical films including individualoptical layers, at least some of which are birefringent, arranged intooptical repeat units through the thickness of the film. Adjacent opticallayers have refractive index relationships that maintain reflectivityand avoid leakage of p-polarized light at moderate to high incidenceangles.

In one aspect, the present disclosure provides an illumination system,including a light source that emits light having a first opticalcharacteristic, and light guides optically coupled to the light source,where each light guide includes an input surface and an output surface.The system further includes emissive material positioned to receivelight from at least one light guide, where the emissive material emitslight having a second optical characteristic when illuminated with lighthaving the first optical characteristic. The system further includes afirst interference reflector positioned between the emissive materialand the output surfaces of the light guides, where the firstinterference reflector substantially transmits light having the firstoptical characteristic and substantially reflects light having thesecond optical characteristic.

In another aspect, the present disclosure provides a display, includingan illumination system and a spatial light modulator. The illuminationsystem includes a light source that emits light having a first opticalcharacteristic, and light guides optically coupled to the light source,where each light guide includes an input surface and an output surface.The system also includes emissive material positioned to receive lightfrom at least one light guide, where the emissive material emits lighthaving a second optical characteristic when illuminated with lighthaving the first optical characteristic. The system also includes afirst interference reflector positioned between the emissive materialand the output surfaces of the light guides, where the firstinterference reflector substantially transmits light having the firstoptical characteristic and substantially reflects light having thesecond optical characteristic. The spatial light modulator is opticallycoupled to the illumination system and includes controllable elementsoperable to modulate at least a portion of light from the illuminationsystem.

In another aspect, the present disclosure provides a method of providingillumination to a desired location, including illuminating at least onelight guide of a plurality of light guides with light having a firstoptical characteristic, where the at least one light guide directs thelight through an output surface; and illuminating a first interferencereflector with light from the output surface of the at least one lightguide, where the first interference reflector substantially transmitslight having the first optical characteristic and substantially reflectslight having a second optical characteristic. The method furtherincludes illuminating emissive material with the light transmitted bythe first interference reflector such that the emissive material emitslight having the second optical characteristic; and directing at least aportion of the light emitted by the emissive material to the desiredlocation.

The above summary of the present disclosure is not intended to describeeach disclosed embodiment or every implementation of the presentdisclosure. The Figures and Detailed Description that follow moreparticularly exemplify illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates one embodiment of an illuminationsystem having a short pass interference reflector.

FIG. 2 schematically illustrates one embodiment of an illuminationsystem having a short pass interference reflector and a long passinterference reflector.

FIG. 3 schematically illustrates one embodiment of an illuminationsystem having a short pass interference reflector and one or moreoptical elements.

FIG. 4 schematically illustrates one embodiment of an illuminationsystem having a long pass interference reflector.

FIG. 5 schematically illustrates one embodiment of an illuminationsystem having a short pass interference reflector, a long passinterference reflector, and an optical cavity.

FIG. 6(A) is a schematic top plan view of one embodiment of anillumination system having an optical cavity that includes one or morefacets.

FIG. 6(B) is a schematic cross-section view of one portion of theoptical cavity of the illumination system of FIG. 6(A).

FIG. 6(C) is a schematic side view of the illumination system of FIG.6(A).

FIG. 6(D) is a schematic side view of another embodiment of anillumination system having an optical cavity that includes one or morefacets.

FIG. 6(E) is a schematic side view of another embodiment of anillumination system having an optical cavity that includes one or morefacets.

FIG. 7 is a schematic top plan view of another embodiment of anillumination system having four optical cavities that each include oneor more facets.

FIG. 8(A) is a schematic top plan view of an embodiment of anillumination system having a short pass interference reflector locatedwithin one or more optical cavities located within a light guide.

FIG. 8(B) is a schematic cross-section view of the illumination systemof FIG. 8A taken along line 8B-8B.

FIG. 9(A) is a schematic side view of an embodiment of an illuminationsystem having one or more optical cavities adjacent an input surface ofa light guide.

FIG. 9(B) is a schematic top plan view of the illumination system ofFIG. 9(a).

FIG. 10(A) is a schematic perspective view of one embodiment of anillumination system having one or more optical cavities adjacent aninput surface of a light guide.

FIG. 10(B) is a schematic cross-section view of the illumination systemof FIG. 10(a) taken along line 10B-10B.

FIG. 11 schematically illustrates an embodiment of an illuminationsystem having a short pass interference reflector positioned betweenemissive material and an output surface of a light guide.

FIG. 12 schematically illustrates an embodiment of an illuminationsystem having a short pass interference reflector positioned adjacent anoutput surface of a light guide and one or more phosphor dots positionedon the short pass interference reflector.

FIG. 13 is a schematic perspective view of one embodiment of anillumination system having a wedge-shaped light guide.

FIG. 14 is a schematic cross-section view of one embodiment of anillumination system having one or more light guides.

FIG. 15 schematically illustrates another embodiment of an illuminationsystem having one or more light guides.

FIG. 16 schematically illustrates a display assembly including anillumination system and a display device.

FIG. 17 schematically illustrates an embodiment of an illuminationsystem having a long pass interference reflector positioned such thatemissive material is between an output surface of a light guide and thelong pass interference reflector.

FIG. 18 is a schematic cross-section view of another embodiment of anillumination system having one or more light guides.

DETAILED DESCRIPTION

The present disclosure provides illumination systems that include alight source, one or more light guides, emissive material, and one ormore interference reflectors. In some embodiments, the illuminationsystems provide white light for various applications. As used herein,the term “white light” refers to light that stimulates red, green, andblue sensors in the human eye to yield an appearance that an ordinaryobserver would consider “white.” Such light may be biased to the red(commonly referred to as warm white light) or to the blue (commonlyreferred to as cool white light). Further, such light can have a colorrendering index of up to 100.

In general, these illumination systems include a light source that emitslight including a first optical characteristic. The systems of thepresent disclosure also include emissive material that emits lighthaving a second optical characteristic when illuminated with lighthaving the first optical characteristic. The first opticalcharacteristic and second optical characteristic may be any suitableoptical characteristic, e.g., wavelength, polarization, modulation,intensity, etc. For example, the first optical characteristic mayinclude a first wavelength region, and the second optical characteristicmay include a second wavelength region that is different than the firstwavelength region. In one exemplary embodiment, the light source mayemit light having a first optical characteristic, where the firstoptical characteristic includes a first wavelength region including UVlight. In this illustrative embodiment, the UV light emitted by thelight source illuminates emissive material, which cause such material toemit light having a second optical characteristic, where the secondoptical characteristic includes a second wavelength region includingvisible light.

Some embodiments of the present disclosure include a short pass (SP)interference reflector. As used herein, the term “short passinterference reflector” refers to a reflector that substantiallytransmits light having a first optical characteristic and substantiallyreflects light having a second optical characteristic. In one exemplaryembodiment, an illumination system includes a SP interference reflectorthat substantially transmits UV light from a light source andsubstantially reflects visible light emitted by emissive material thathas been illuminated by the transmitted UV light.

Further, in some embodiments, the illumination systems include a longpass (LP) interference reflector. As used herein, the term “long passinterference reflector” refers to a reflector that substantiallytransmits light having a second optical characteristic and substantiallyreflects light having a first optical characteristic. For example, inone exemplary embodiment, an illumination system includes a LPinterference reflector that substantially transmits visible lightemitted by emissive material and substantially reflects UV light from alight source that had illuminated the emissive material.

In general, when the first optical characteristic and second opticalcharacteristic are associated with wavelength, the emissive materials ofthe present disclosure may down-convert shorter wavelength light (e.g.,UV light) to longer wavelength light (e.g., visible light).Alternatively, it is also possible to up-convert infrared radiation tovisible light. For example, up-converting phosphors are well known inthe art and typically use two or more infrared photons to generate 1visible photon. Infrared LEDs needed to excite such phosphors have alsobeen demonstrated and are very efficient. Visible light sources that usethis process can be made more efficient with the addition of LPinterference reflectors and/or SP interference reflectors, although thefunctions of each are reversed in this case compared to thedown-converting phosphor systems. A SP interference reflector can beused to direct IR light towards the phosphor while transmitting thevisible light, and an LP interference reflector can be placed such thatthe phosphor is between the LED and the LP interference reflector, wherethe LP interference reflector directs emitted visible light outwardtowards the intended system or user.

Although the exemplary embodiments of the present disclosure generallyassociate the first optical characteristic and second opticalcharacteristic with wavelength, it is understood that such exemplaryembodiments can also associate the first optical characteristic andsecond optical characteristic with other suitable characteristics oflight, e.g., polarization, modulation, intensity, etc. For example, a SPinterference reflector may be selected such that it substantiallytransmits light of a first polarization while the LP interferencereflector substantially transmits light of a second polarization.

The illumination systems of the present disclosure may be used in anysuitable application. For example, in some embodiments, an illuminationsystem may be used as a light source for displays, light fixtures,headlamps, signs, etc.

In some embodiments, one or both of the SP interference reflector and LPinterference reflector include polymeric multilayer optical films.Polymeric multilayer optical films are films that have tens, hundreds,or thousands of alternating layers of at least a first and secondpolymer material. Such layers have thicknesses and refractive indicesthat are selected to achieve a desired reflectivity in a desired portionof the spectrum, such as a reflection band limited to UV wavelengths ora reflection band limited to visible wavelengths. See, e.g., U.S. Pat.No. 5,882,774 (Jonza et al.). The polymeric multilayer optical films canbe processed so that adjacent layer pairs have matching ornear-matching, or deliberately mismatched refractive indices associatedwith a z-axis normal to the film such that the reflectivity of eachinterface between adjacent layers, for p-polarized light, decreasesslowly with angle of incidence, is substantially independent of angle ofincidence, or increases with angle of incidence away from the normal.Hence, such polymeric multilayer optical films can maintain highreflectivity levels for p-polarized light even at highly obliqueincidence angles, thereby reducing the amount of p-polarized lighttransmitted by the reflective films compared to conventional inorganicisotropic stack reflectors. In some embodiments, the polymeric materialsand processing conditions are selected so that, for each pair ofadjacent optical layers, the difference in refractive index along thez-axis (parallel to the thickness of the film) is no more than afraction of the refractive index difference along the x- or y-(in-plane) axes, the fraction being 0.5, 0.25, or even 0.1. Therefractive index difference along the z-axis can be of the same oropposite sign as the in-plane refractive index differences.

Such polymeric multilayer optical films can be formed into any suitableshape as is further described herein. For example, polymeric multilayeroptical film can be permanently deformed by embossing, thermoforming, orother known techniques to have a 3-dimensional shape such as a portionof a paraboloid, a sphere, or an ellipsoid. See, e.g., U.S. PatentApplication Publication No. 2002/0154406 (Merrill et al.). See also U.S.Pat. No. 5,540,978 (Schrenk).

A wide variety of polymer materials are suitable for use in multilayeroptical films for illumination systems. In certain applicationsaccording to various embodiments of the disclosure, it is desirable thatthe multilayer optical film includes alternating polymer layers composedof materials that resist degradation when exposed to UV light, e.g., apolymer pair of polyethylene terephthalate(PET)/co-polymethylmethacrylate (co-PMMA). The UV stability of polymericreflectors can also be increased by the incorporation of non-UVabsorbing light stabilizers such as hindered amine light stabilizers(HALS). In some cases, the polymeric multilayer optical film can alsoinclude transparent metal or metal oxide layers. See, e.g., PCTPublication WO 97/01778 (Ouderkirk et al.). In applications that useparticularly high intensity UV light that would unacceptably degradeeven robust polymer material combinations, it may be beneficial to useinorganic materials to form the multilayer optical films. The inorganicmaterial layers can be isotropic or can be made to exhibit formbirefringence as described, e.g., in PCT Publication WO 01/75490 (Weber)and thus have the beneficial refractive index relationships that yieldenhanced p-polarization reflectivity as described herein.

In general, the interference reflectors described herein includereflectors that are formed of organic, inorganic, or a combination oforganic and inorganic materials. The interference reflector can be amultilayer interference reflector. The interference reflector can be aflexible interference reflector. A flexible interference reflector canbe formed from polymeric, non-polymeric materials, or polymeric andnon-polymeric materials. Exemplary films including a polymeric andnon-polymeric material are disclosed in U.S. Pat. Nos. 6,010,751 (Shawet al.); 6,172,810 (Fleming et al.); and EP 733,919A2 (Shaw et al.).

The interference reflectors described herein can be formed fromflexible, plastic, or deformable materials and can itself be flexible,plastic, or deformable. These flexible interference reflectors can bedeflected or curved and still retain their pre-deflection opticalproperties.

Known self-assembled periodic structures, such as cholesteric reflectingpolarizers and certain block copolymers, are considered to be multilayerinterference reflectors for purposes of this disclosure. Cholestericmirrors can be made using a combination of left and right handed chiralpitch elements.

In some embodiments of the present disclosure, the interferencereflectors can be selected to substantially transmit or partiallytransmit light having a selected optical characteristic.

For example, a LP interference reflector that partially transmits bluelight can be used in combination with a thin yellow phosphor layer inorder to direct some blue light from a light source back onto thephosphor layer after the first pass through the phosphor.

In addition to providing reflection of blue light and UV light, afunction of the multilayer optical film can be to block transmission ofUV light so as to prevent degradation of subsequent elements inside oroutside the illumination system, including prevention of human eyedamage. In some embodiments, a UV absorber may be included on the sideof the UV reflector furthest away from the light source. This UVabsorber can be in, on, or adjacent to the multilayer optical film.

Although the interference reflectors of the present disclosure mayinclude any suitable material or materials, an all polymer constructioncan offer several manufacturing and cost benefits. If high temperaturepolymers with high optical transmission and large index differentialsare utilized in the interference reflectors, then an environmentallystable reflector that is both thin and very flexible can be manufacturedto meet the optical needs of SP and LP interference reflectors. In someembodiments, coextruded multilayer interference reflectors as taught,e.g., in U.S. Pat. No. 6,531,230 (Weber et al.), can provide precisewavelength selection as well as large area, cost effectivemanufacturing. The use of polymer pairs having high index differentialsallows the construction of very thin, highly reflective mirrors that arefreestanding, i.e., have no substrate but are still easily processed.Alternatively, the interference reflectors of the present disclosure maybe formed by casting as is described, e.g., in U.S. Pat. No. 3,711,176(Alfrey, Jr. et al.).

An all polymeric interference reflector can be thermoformed into variousthree-dimensional shapes, e.g., hemispherical domes (as is furtherdescribed herein). However, care must be taken to control the thinningto the correct amount over the entire surface of the dome to create thedesired angular performance. Interference reflectors having a simpletwo-dimensional curvature are easier to create than three-dimensional,compound shaped interference reflectors. In particular, any thin andflexible interference reflector can be bent into a two-dimensionalshape, e.g., a part of a cylinder, in this case an all polymericinterference reflector is not needed. Multilayer inorganic interferencereflectors on thin polymeric substrates can be shaped in this manner, aswell as inorganic multilayers on glass substrates that are less than 200μm in thickness. The latter may have to be heated to temperatures nearthe glass transition point to obtain a permanent shape with low stress.

Optimum bandedges for SP and LP interference reflectors will depend onthe emission spectra of both the light source and the emissive materialin the system. In an illustrative embodiment, for a SP interferencereflector, substantially all of the emission from the light sourcepasses through the SP interference reflector to excite the emissivematerial, and substantially all of the emissions directed back towardthe light source are reflected by the SP interference reflector so theydo not enter the light source or its base structure where they could beabsorbed. For this reason, the short pass defining bandedge of the SPinterference reflector is placed in a region between the averageemission wavelength of the light source and the average emissionwavelength of the emissive material. In an illustrative embodiment, theSP interference reflector is placed between the light source and theemissive material. If, however, the SP interference reflector is planar,the emissions from a light source can strike the SP interferencereflector at a variety of angles normal to a surface of the SPinterference reflector, and at some angle of incidence be reflected bythe SP interference reflector and fail to reach the emissive material.Unless the interference reflector is curved to maintain a nearlyconstant angle of incidence, one may desire to place the design bandedgeat a wavelength larger than the midpoint of the emissive material andthe light source emission curves to optimize the overall systemperformance. In particular, very little emissive material emission isdirected to the interference reflector near zero degrees angle ofincidence (i.e., normal to a surface of the interference reflector)because the included solid angle is very small.

In another illustrative embodiment, LP interference reflectors areplaced opposite the emissive material from the light source to recyclethe light source light back to the emissive material to improve systemefficiency. In an illustrative embodiment, a LP interference reflectormay be omitted if the light source emissions are in the visible spectrumand large amounts are needed to balance the color output of the emissivematerial. However, a LP interference reflector that partially transmitsshorter wavelength light, e.g., blue light, can be used to optimize theangular performance of a blue-light source/yellow-phosphor system viathe spectral angle shift that would pass more blue light at higherangles than at normal incidence.

In a further illustrative embodiment, the LP interference reflector iscurved to maintain a nearly constant angle of incidence of the emittedlight from the light source on the LP interference reflector. In thisembodiment, the emissive material and the light source both face oneside of the LP interference reflector. At high angles of incidence, a LPinterference reflector having a substantially planar shape may notreflect shorter wavelength light. For this reason, the long wavelengthbandedge of the LP interference reflector can be placed at as long awavelength as possible while blocking as little of the emissive materialemission as possible. Again, the bandedge placement can be changed tooptimize the overall system efficiency.

In some embodiments, the multilayer interference reflectors describedherein may have a lateral thickness gradient, i.e., a thickness thatdiffers from one cross-section of the reflector to another cross-sectionof the reflector. These reflectors may have thicker interference layersas the emitted light angle of incidence increases toward an outer regionof the multilayer reflector. Increasing the reflector thickness at theouter region of the reflector compensates for band shifting, since thereflected wavelength is proportional to the optical thickness of thehigh and low index interference layers and the incidence angle.

FIG. 1 schematically illustrates one embodiment of an illuminationsystem 10. The system 10 includes a light source 20 and a light guide 12having an output surface 14. In some embodiments, the light guide 12 canalso include an input surface 16. The system 10 also includes a firstinterference reflector 30 positioned between the light source 20 and theoutput surface 14 of the light guide 12. Positioned between the firstinterference reflector 30 and the output surface 14 of the light guide12 is emissive material 40.

The light source 20 can include any suitable light source or lightsources, e.g., electroluminescent devices, cold cathode fluorescentlights, electrodeless fluorescent lamps, LEDs, organicelectroluminescent devices (OLEDs), polymer LEDs, laser diodes, arclamps, etc. As used herein, the term “LED” refers to a diode that emitslight, whether visible, ultraviolet, or infrared, whether coherent orincoherent. The term as used herein also includes incoherentepoxy-encased semiconductor devices marketed as “LEDs,” whether of theconventional or super-radiant variety. The term as used herein alsoincludes semiconductor laser diodes.

In some embodiments, the light source 20 can be positioned adjacent oneor more sides of the light guide 12, and/or one or more major surfacesof the light guide 12. As illustrated in FIG. 1, the light source 20 ispositioned adjacent the input surface 16. Although FIG. 1 illustratesillumination system 10 as having one light source 20, illuminationsystem 10 may include two or more light sources positioned adjacent thesame or other input surfaces of the light guide 12.

The light source 20 emits light having a first optical characteristic.Any suitable optical characteristic may be selected. In someembodiments, the first optical characteristic can include a firstwavelength region. For example, the light source 20 may emit UV light.As used herein, the term “UV light” refers to light having a wavelengthin a range from about 150 nm to about 425 nm. In another example, thelight source 20 may emit blue light.

In some embodiments, the light source 20 includes one or more LEDs. Forexample, the one or more LEDs can emit UV light and/or blue light. Bluelight also includes violet and indigo light. LEDs include spontaneousemission devices as well as devices using stimulated or super radiantemission, including laser diodes and vertical cavity surface emittinglaser diodes.

The light guide 12 of system 10 may include any suitable light guide,e.g., hollow or solid light guide. Although the light guide 12 isillustrated as being planar in shape, the light guide 12 may take anysuitable shape, e.g., wedge, cylindrical, planar, conical, complexmolded shapes, etc. Further, the input surface 16 and/or the outputsurface 14 of the light guide 12 may include any suitable shapes, e.g.,those described above for the shape of the light guide 12. It may bepreferred that the light guide 12 is configured to direct light throughits output surface 14. Further, the light guide 12 may include anysuitable material or materials. For example, the light guide 12 mayinclude glass; acrylates, including polymethylmethacrylate, polystyrene,fluoropolymers; polyesters including polyethylene terephthalate (PET),polyethylene naphthalate (PEN), and copolymers containing PET or PEN orboth; polyolefins including polyethylene, polypropylene, polynorborene,polyolefins in isotactic, atactic, and syndiotactic sterioisomers, andpolyolefins produced by metallocene polymerization. Other suitablepolymers include polyetheretherketones and polyetherimides.

The illumination system 10 also includes a first interference reflector30 positioned between the light source 20 and the output surface 14 ofthe light guide 12. In the embodiment illustrated in FIG. 1, the firstinterference reflector 30 is a SP interference reflector, i.e., itsubstantially transmits light having the first optical characteristicfrom the light source 20 and substantially reflects light having asecond optical characteristic. For example, as is further describedherein, the emissive material 40 may emit visible light when illuminatedwith UV or blue light from the light source 20. In such an embodiment,the first interference reflector 30 may be selected such that itsubstantially transmits UV light and substantially reflects visiblelight. In other embodiments, the emissive material 40 may emit infraredlight when illuminated with light from the light source 20. In suchembodiments, the first interference reflector 30 may be selected suchthat it substantially transmits light from the light source 20 andsubstantially reflects infrared light.

The first interference reflector 30 may be positioned in any suitablelocation between the light source 20 and the output surface 14 of thelight guide 12. In some embodiments, the first interference reflector 30may be positioned on the input surface 16 of the light guide 12, withinthe light guide 12, or on the light source 20.

The first interference reflector 30 may include any suitableinterference reflector or reflectors described herein. Further, thefirst interference reflector 30 may take any suitable shape, e.g.,hemispherical, cylindrical, planar, etc.

The first interference reflector 30 can be formed of a material thatresists degradation when exposed to UV, blue, or violet light, such asdiscussed herein. In general, the multilayer reflectors discussed hereincan be stable under high intensity illumination for extended periods oftime. High intensity illumination can be generally defined as a fluxlevel from 1 to 100 Watt/cm². Suitable illustrative polymeric materialscan include UV resistant material formed from, for example, acrylicmaterial, PET material, PMMA material, polystyrene material,polycarbonate material, THV material available from 3M Company (St.Paul, Minn.), and combinations thereof. These materials and PEN materialcan be used with light sources that emit blue light.

The illumination system 10 also includes emissive material 40 positionedbetween the first interference reflector 30 and the output surface 14 ofthe light guide 12. The emissive material 40 emits light having a secondoptical characteristic when illuminated with light having the firstoptical characteristic from the light source 20. The second opticalcharacteristic may be any suitable optical characteristic, e.g.,wavelength, polarization, modulation, intensity, etc. In someembodiments, the light emitted by the emissive material 40 may include asecond wavelength region when the emissive material 40 is illuminatedwith light emitted by the light source 20 that includes a firstwavelength region. For example, in some embodiments, the emissivematerial 40 may emit visible light when illuminated with UV or bluelight form the light source 20. As used herein, the term “visible light”refers to light that is perceptible to the unaided eye, e.g., generallyin a wavelength range of about 400 nm to about 780 nm. In otherembodiments, the emissive material 40 may emit visible light and/orinfrared light. As used herein, the term “infrared light” refers tolight in a wavelength range of 780 nm to 2500 nm.

In general, the embodiments disclosed herein are operative with avariety of emissive materials. In some embodiments, suitable phosphormaterials may be used. Such phosphor materials are typically inorganicin composition, having excitation wavelengths in the 150-1100 nm range.A phosphor blend can comprise phosphor particles in the 1-25 μm sizerange dispersed in a binder such as silicone, fluoropolymer, epoxy,adhesive, or another polymeric matrix, which can then be applied to asubstrate, such as an LED or a film. Phosphors include rare-earth dopedgarnets, silicates, and other ceramics. In other embodiments, theemissive materials can also include organic fluorescent materials,including fluorescent dyes and pigments, sulfides, aluminates,phosphates, nitrides. See, e.g., Shionoya et al., Phosphor Handbook, CRCPress, Boca Raton, Fla. (1998).

In embodiments that utilize emissive materials 40 having a narrowemission wavelength range, a mixture of emissive materials can beformulated to achieve the desired color balance, as perceived by theviewer, for example a mixture of red-, green- and blue-emittingmaterials. In other embodiments, emissive materials having broaderemission bands can be useful for mixtures having higher color renderingindices. In some embodiments, the emissive materials can have fastradiative decay rates.

The emissive material 40 can be formed in a continuous or discontinuouslayer. The emissive material 40 can be a uniform or non-uniform pattern.The emissive material 40 can include regions having a small area, e.g.,“dots,” each having an area in plan view of less than 10000 μm². In anillustrative embodiment, the dots can each be formed from a phosphorthat emits longer wavelength light having one or more different peakwavelengths. For example, at least one dot can include a first emissivematerial that emits a peak wavelength in the red region, and at leastanother phosphor dot can include a second emissive material that emits apeak wavelength in the blue region. The dots emitting visible lighthaving a plurality of peak wavelengths can be arranged and configured inany uniform or non-uniform manner as desired. For example, the emissivematerial 40 can include dots in a pattern having a non-uniform densitygradient along a surface or an area. The dots can have any regular orirregular shape and need not be round in plan view. In addition,emissive material 40 can be in a co-extruded skin layer of a multilayeroptical film.

Structured emissive materials can be configured in several ways toprovide benefits in performance as described herein. When multiplephosphor types are used to provide broader or fuller spectral output,light from shorter wavelength phosphors can be re-absorbed by otherphosphors. Patterns including isolated dots, lines, or isolated regionsof each phosphor type can reduce the amount of re-absorption.

Multilayer emissive material structures can also reduce absorption. Forexample, layers of each emissive material may be formed in sequence,with the longest wavelength emitter nearest the excitation source. Lightemitted nearer the emitter will, on average, undergo multiple scatteringwithin the total emissive material to a greater extent than lightemitted near the output surface of the emissive material. Since theshortest wavelength emitted is most prone to both scattering andre-absorption, it may be preferred to locate the shortest wavelengthemissive material nearest the output surface of the emissive material.In addition, it may be preferred to use different thicknesses for eachlayer to compensate for the progressively lower intensity of theexcitation light as it propagates through the multilayer structure. Foremissive materials with similar absorption and emission efficiency,progressively thinner layers from excitation to output side wouldprovide compensation for the decreasing excitation intensity in eachlayer. In some embodiments, one or more SP interference reflectors maybe positioned between the different emissive material layers to reducethe emitted phosphor light that is scattered backward and re-absorbed bylayers earlier in the sequence.

Non-scattering emissive materials can provide enhanced light output incombination with multilayer optical films. For example, non-scatteringphosphor layers can include conventional phosphors in an index-matchedbinder (e.g., a binder with high index inert nanoparticles), nanosizeparticles of conventional phosphor compositions (e.g., where particlesizes are small and negligibly scatter light), or quantum dot emissivematerials. Quantum dot emissive materials are light emitters based onsemiconductors having low band gaps, e.g., cadmium sulfide, cadmiumselenide or silicon, where the particles are sufficiently small so thatthe electronic structure is influenced and controlled by the particlesize. Hence, the absorption and emission spectra are controlled via theparticle size. See, e.g., U.S. Pat. No. 6,501,091 (Bawendi et al.).

The emissive material 40 may be positioned in any suitable locationbetween the first interference reflector 30 and the output surface 14 ofthe light guide 12. In some embodiments, the emissive material 40 may bepositioned on the input surface 16 of the light guide 12. Alternatively,the emissive material 40 may be placed within the light guide 12. Inother embodiments, the emissive material 40 may be dispersed within thelight guide 12. In other embodiments, the emissive material 40 may bepositioned on an output surface 32 of the first interference reflector30. Any suitable technique may be used to position the emissive material40 on the first interference reflector 30, e.g., those techniquesdescribed in co-owned and co-pending U.S. Pat. No. 7,118,438 (Ouderkirket al.). For example, the emissive material 40 can be disposed or coatedon the first interference reflector 30. The emissive material 40 can belaminated, as a solid layer, adjacent the first interference reflector30. In addition, the emissive material 40 and the first interferencereflector 30 can be thermoformed sequentially or simultaneously. Theemissive material 40 can be compressible, elastomeric, and can even becontained in a foamed structure.

In some embodiments, the system 10 can also include a TIR promotinglayer positioned on the emissive material 40 between the emissivematerial 40 and the first interference reflector 30. The TIR promotinglayer may include any suitable material or materials that provide arefractive index that is lower than the refractive index of the binderin the emissive material 40. The TIR promoting layer may, in someembodiments, be an air gap. Such an air gap enables total internalreflection of light traversing at high incidence angles in the emissivematerial 40. In other embodiments, the TIR promoting layer may be amicrostructured layer having a microstructured surface. Themicrostructured surface can be characterized by a single set of linearv-shaped grooves or prisms, multiple intersecting sets of v-shapedgrooves that define arrays of tiny pyramids, one or more sets of narrowridges, and so forth. When the microstructured surface of such a film isplaced against another flat film, air gaps are formed between theuppermost portions of the microstructured surface and the flat film.

Certain types of emissive materials can produce heat, for example, whenconverting light from a first wavelength region to a second wavelength.The presence of an air gap near the emissive material 40 maysignificantly reduce heat transmission from the emissive material 40 tosurrounding materials. The reduced heat transfer can be compensated forin other ways, such as by providing a layer of glass or transparentceramic near the emissive material 40 that can remove heat laterally.

In general, the light source 20 emits light having a first opticalcharacteristic, at least a portion of which illuminates the firstinterference reflector 30. In turn, the first interference reflector 30substantially transmits the light from the light source 20. At least aportion of the transmitted light illuminates the emissive material 40.The emissive material 40 emits light having a second opticalcharacteristic when illuminated with light having the first opticalcharacteristic. Generally, the emissive material 40 may emit light inany direction. In other words, some light may be emitted back toward thelight source 20, and some light may be emitted toward the light guide12. Light emitted by the emissive material 40 that illuminates the firstinterference reflector 30 is substantially reflected such that the lightdoes not reach the light source 20 where it can be absorbed. The lightguide 12 directs at least a portion of the light emitted by the emissivematerial 40 through the output surface 14 where it can then be directedto a desired location using any suitable technique.

Some embodiments of illumination systems of the present disclosure mayinclude more than one interference reflector. For example, FIG. 2schematically illustrates one embodiment of an illumination system 100that includes a light guide 112 having an output surface 114 and aninput surface 116, and a light source 120. The system 100 also includesa first interference reflector 130 positioned between the light source120 and the output surface 114 of the light guide 112, and emissivematerial 140 positioned between the first interference reflector 130 andthe output surface 114. All of the design considerations andpossibilities described herein with respect to the light guide 12, thelight source 20, the first interference reflector 30, and the emissivematerial 40 of the embodiment illustrated in FIG. 1 apply equally to thelight guide 112, the light source 120, the first interference reflector130, and the emissive material 140 of the embodiment illustrated in FIG.2.

One difference between the system 10 of FIG. 1 and the system 100 ofFIG. 2 is that system 100 includes a second interference reflector 150positioned such that the emissive material 140 is between the firstinterference reflector 130 and the second interference reflector 150. Insome embodiments, the second interference reflector 150 is a LPinterference reflector, i.e., a reflector that substantially transmitslight having a second optical characteristic and substantially reflectslight having a first optical characteristic. For example, in someembodiments, the emissive material 140 may emit visible light (i.e., thesecond optical characteristic) when illuminated with UV or blue light(i.e., the first optical characteristic). In such embodiments, thesecond interference reflector 150 may be selected such that itsubstantially transmits visible light and substantially reflects UV orblue light. In other embodiments, the emissive material 140 may emitinfrared light when illuminated with UV or blue light. In theseembodiments, the second interference reflector 150 may be selected suchthat it substantially transmits infrared light and substantiallyreflects UV or blue light.

The second interference reflector 150 may include any suitableinterference reflector or reflectors described herein. Further, thesecond interference reflector 150 may take any suitable shape, e.g.,hemispherical, cylindrical, or planar.

The second interference reflector 150 may be positioned in any suitablelocation between the emissive material 140 and the output surface 114 ofthe light guide 112. In some embodiments, the second interferencereflector 150 may be positioned on the input surface 116 of the lightguide 112. In other embodiments, the second interference reflector 150may be positioned within the light guide 112. In some embodiments, theemissive material 140 may be positioned on the second interferencereflector 150 as is further described, e.g., in co-owned and co-pendingU.S. Patent Application Publication No. 2004/0145913 (Ouderkirk et al.).Alternatively, the first interference reflector 130, emissive material140, and second interference reflector 150 may form an assembly wherethe emissive material 140 is in contact with both the first interferencereflector 130 and the second interference reflector 150. Any suitabletechnique may be used to form such an assembly, e.g., those techniquesas described in co-owned and co-pending U.S. Pat. No. 7,118,438(Ouderkirk et al.).

The presence of the first interference reflector 130 and secondinterference reflector 150 can enhance the efficiency of theillumination system 100. The second interference reflector 150 reflectsat least a portion of the light that is not absorbed by the emissivematerial 140, and that would otherwise be wasted, back into the emissivematerial 140. This increases the effective path length of the light fromthe light source 120 through the emissive material 140, therebyincreasing the amount of light absorbed by the emissive material 140 fora given thickness of the emissive material layer or layers. Therecycling of the light from the light source 120 also allows use ofthinner layers of emissive material 140 for efficient light conversion.

In general, at least a portion of light having a first opticalcharacteristic emitted by the light source 120 illuminates the firstinterference reflector 130, which substantially transmits such light. Atleast a portion of light transmitted by the first interference reflector130 illuminates the emissive material 140. When illuminated with lighthaving the first optical characteristic, the emissive material 140 emitslight having a second optical characteristic. At least a portion of thelight emitted by the emissive material 140 illuminates the secondinterference reflector 150, which substantially transmits light havingthe second optical characteristic. At least a portion of the transmittedlight enters the light guide 112 and is directed through the outputsurface 114 by the light guide 112. Any light from the light source 120that illuminates the second interference reflector 150 is substantiallyreflected towards the emissive material 140 where it may excite theemissive material 140 causing further light emission. In addition, lightemitted by the emissive material 140 that illuminates the firstinterference reflector 130 is substantially reflected back toward thesecond interference reflector 150 and/or the light guide 112.

The illumination systems of the present disclosure may include one ormore optical elements. For example, FIG. 3 schematically illustrates anillumination system 200 that includes one or more optical elements 260.The system 200 further includes a light guide 212 having an outputsurface 214 and an input surface 216, and a light source 220. The system200 also includes a first interference reflector 230 positioned betweenthe light source 220 and the output surface 214 of the light guide 212,and emissive material 240 positioned between the first interferencereflector 230 and the output surface 214 of the light guide 212. All ofthe design considerations and possibilities described herein withrespect to the light guide 12, the light source 20, the firstinterference reflector 30, and the emissive material 40 of theembodiment illustrated in FIG. 1 apply equally to the light guide 212,the light source 220, the first interference reflector 230, and theemissive material 240 of the embodiment illustrated in FIG. 3. Thesystem 200 may also include one or more additional interferencereflectors (e.g., a LP interference reflector) as is further describedherein.

The one or more optical elements 260 may be positioned between theemissive material 240 and the output surface 214 of the light guide 212,between the light source 220 and the first interference reflector 230,between the first interference reflector 230 and the emissive material240 and/or adjacent the output surface 214 of the light guide 212. Theone or more optical elements 260 can include any suitable opticalelement or elements, e.g., optical coupling agents such as adhesives orindex matching fluids or gels, optical brightness enhancing films suchas BEF (available from 3M Company), and short-wavelength absorbingmaterials such as ultraviolet light absorbing dyes and pigments,reflective polarizing films such as DBEF (also available from 3MCompany), diffusers, and combinations thereof. In some embodiments, theone or more optical elements 260 are configured to control the angle oflight emitted by the emissive material 240 that is directed into thelight guide 212.

In some embodiments, the one or more optical elements 260 may includeone or more reflective polarizers. In general, a reflective polarizercan be disposed adjacent the emissive material 240. The reflectivepolarizer allows light of a preferred polarization to be transmitted,while reflecting the other polarization. The emissive material 240 andother film components known in the art can depolarize the polarizedlight reflected by a reflective polarizer, and either by the reflectionof the emissive material 240, or emissive material 240 in combinationwith the first interference reflector 230, light can be recycled andincrease the polarized light brightness of the system 200. Suitablereflective polarizers include, for example, cholesteric reflectivepolarizers, cholesteric reflective polarizers with a ¼ wave retarder,wire grid polarizers, or a variety of reflective polarizers availablefrom 3M Company, including DBEF (i.e., a specularly reflectivepolarizer), and DRPF (i.e., a diffusely reflective polarizer). Thereflective polarizer preferably polarizes light over a substantial rangeof wavelengths and angles emitted by the emissive material 240, and inthe case where the light source 220 emits blue light, may reflect theblue light as well.

Although the one or more optical elements 260 are illustrated in FIG. 3as being outside of the light guide 212, the one or more opticalelements 260 may be positioned on or inside the light guide 212. In someembodiments, the one or more optical elements 260 may be positioned onthe emissive material 240. If a LP interference reflector is included insystem 300 and positioned between the emissive material 240 and theoutput surface 214, then the one or more optical elements 260 may bepositioned on the LP interference reflector.

In some embodiments, an illumination system may include a LP reflectorwithout a SP reflector. For example, FIG. 4 schematically illustratesanother embodiment of an illumination system 300. The system 300includes a light guide 312 having an output surface 314 and an inputsurface 316, and a light source 320. The system 300 further includesemissive material 340 positioned between the light source 320 and theoutput surface 314 of the light guide, and an interference reflector 350positioned between the emissive material 340 and the output surface 314of the light guide 312. All of the design considerations andpossibilities described herein with respect to the light guide 112, thelight source 120, the emissive material 140, and the second interferencereflector 150 of the embodiment illustrated in FIG. 2 apply equally tothe light guide 312, the light source 320, the emissive material 340,and the interference reflector 350 of the embodiment illustrated in FIG.4.

Although depicted as being positioned outside the light guide 312, theinterference reflector 350 can be positioned in any suitable positionbetween the emissive material 320 and the output surface 314 of thelight guide 312. For example, the interference reflector 350 may bepositioned on the input surface 316 of the light guide 312, or insidethe light guide 312. In some embodiments, the interference reflector 350is positioned on the emissive material 340.

Further, in some embodiments, system 300 may include one or more opticalelements positioned between the light source 320 and the emissivematerial 340, between the emissive material 340 and the interferencereflector 350, between the interference reflector 350 and the outputsurface 314 of the light guide 312, and/or adjacent the output surface314 of the light guide 312 (e.g., one or more optical elements 360 ofFIG. 3).

In general, the light source 320 emits light having a first opticalcharacteristic, at least a portion of which illuminates the emissivematerial 340. When illuminated with light having the first opticalcharacteristic, the emissive material 340 emits light having a secondoptical characteristic. At least a portion of the light emitted by theemissive material 340 illuminates the interference reflector 350. Theinterference reflector 350 substantially transmits light having thesecond optical characteristic and substantially reflects light havingthe first optical characteristic. At least a portion of the transmittedlight is directed by the light guide 312 through the output surface 314of the light guide 312. Any light emitted by the light source 320 thatis not converted by the emissive material 340 is substantially reflectedby the interference reflector 350 and directed back toward the emissivematerial 340 where it can be converted. Light directed through theoutput surface 314 can be directed to a desired location using anysuitable technique.

Some light sources that may be utilized in the illumination systems ofthe present disclosure emit light in a broad emission cone. For example,some LEDs emit light in a hemispherical pattern having a solid angle of2π steradians or greater. Some embodiments of the present disclosureprovide non-imaging optical devices to collect and/or direct excitationlight from the light source into the light guide.

For example, FIG. 5 is a schematic perspective view of one embodiment ofan illumination system 400. The system 400 is similar to theillumination system 100 of FIG. 2. System 400 includes a light guide 412having an output surface 414 and an input surface 416, and a lightsource 420. The system 400 also includes a first interference reflector430 positioned between the light source 420 and the output surface 414of the light guide 412, emissive material 440 positioned between thefirst interference reflector 430 and the output surface 414, and asecond interference reflector 450 positioned between the emissivematerial 440 and the output surface 414. All of the designconsiderations and possibilities described herein with respect to thelight guide 112, the light source 120, the first interference reflector130, the emissive material 140, and the second interference reflector150 of the embodiment illustrated in FIG. 2 apply equally to the lightguide 412, the light source 420, the first interference reflector 430,the emissive material 440, and the second interference reflector 450 ofthe embodiment illustrated in FIG. 5.

One difference between the system 100 of FIG. 2 and the system 400 ofFIG. 5 is that system 400 also includes an optical cavity 470 opticallycoupled to the light source 420, i.e., light from the light source 420can be directed into the optical cavity 470. When two or more devicesare optically coupled, such devices are in the same optical path and candirect light to each other using any suitable technique, e.g.,reflection, transmission, emission, etc. The optical cavity 470 isconfigured to direct light emitted by the light source 420 toward thefirst interference reflector 430. The optical cavity 470 may bepositioned in any suitable location. In some embodiments, the opticalcavity 470 may be positioned in contact with the first interferencereflector 430. In some embodiments, a TIR promoting layer or layers maybe positioned between the optical cavity 470 and the first interferencereflector 430 as is further described herein.

The optical cavity 470 may take any suitable shape, e.g., elliptical,wedge, rectangular, trapezoidal, etc. It may be preferred that theoptical cavity 470 take a parabolic shape.

The optical cavity 470 may be made using any suitable material ormaterials. In some embodiments, the optical cavity 470 may include abroadband interference reflector 472. The broadband interferencereflector 472 may be positioned on an optically clear body to formoptical cavity 470. Such an optically clear body may be made of anysuitable material or materials, e.g., glass; acrylates, includingpolymethylmethacrylate, polystyrene, fluoropolymers; polyestersincluding polyethylene terephthalate (PET), polyethylene naphthalate(PEN), and copolymers containing PET or PEN or both; polyolefinsincluding polyethylene, polypropylene, polynorborene, polyolefins inisotactic, atactic, and syndiotactic sterioisomers, and polyolefinsproduced by metallocene polymerization. Other suitable polymers includepolyetheretherketones and polyetherimides. In some embodiments, thebroadband interference reflector 472 may be formed into the desiredshape to form optical cavity 470. The broadband interference reflector472 may be made using any suitable material or materials and using anysuitable techniques, such as those materials and techniques described,e.g., in U.S. Pat. No. 5,882,774 (Jonza et al.).

In some embodiments, the optical cavity 470 may be solid. Alternatively,the optical cavity 470 may be filled with any suitable medium, e.g.,gas, or liquid.

The optical cavity 470 is formed such that the light source 420 emitslight into the optical cavity 470. Any suitable technique may be usedsuch that light is directed into the optical cavity 470. For example,the light source 420 may be placed within the optical cavity 470.Alternatively, the light source 420 may be optically coupled to theoptical cavity 470 via one or more openings or ports formed in theoptical cavity 470.

In some embodiments, the optical cavity 470 may include one or moreapertures (not shown) that allow light from the light source 420 toilluminate the first interference reflector 430. In one exemplaryembodiment, the optical cavity 470 may include an elongated aperturethat extends along at least a portion of the length of the opticalcavity 470. The elongated aperture may be positioned adjacent the firstinterference reflector 430. In some embodiments, the optical cavity 470may include diffusers or facets that may direct light substantiallynormal to a major surface of the first interference reflector 430.

In general, the light source 420 emits light having a first opticalcharacteristic, which is directed by the optical cavity 470 toward thefirst interference reflector 430. The first interference reflector 430substantially transmits light from the light source 420 such that itilluminates the emissive material 440. At least a portion of the lightthat is not transmitted by the first interference reflector 430 iscollected by the optical cavity 470 and redirected toward the firstinterference reflector 430. When illuminated with light having the firstoptical characteristic, the emissive material 440 emits light having asecond optical characteristic. At least a portion of light emitted bythe emissive material 440 illuminates the second interference reflector450. Any light emitted by the emissive material 440 toward the opticalcavity 470 is substantially reflected by the first interferencereflector 430 back toward the emissive material 440. Light emitted bythe emissive material 440 that may be transmitted by the firstinterference reflector 430 is collected by the optical cavity 470 anddirected back toward the first interference reflector 430. The secondinterference reflector 450, which substantially transmits light havingthe second optical characteristic and substantially reflects lighthaving the first optical characteristic, substantially transmits lightemitted by the emissive material 440 toward the input surface 416 of thelight guide 412 where it is directed through the output surface 414 andsubsequently to a desired location. Any light from the light source 420that illuminates the second interference reflector 450 is substantiallyreflected back toward the emissive material 440 where it may beconverted to light having the second optical characteristic.

Although FIG. 5 illustrates illumination system 400 as including a firstinterference reflector 430, in some embodiments, the system 400 may notinclude a first interference reflector. In such embodiments, the opticalcavity 470 is positioned adjacent the emissive material 440 such that atleast a portion of excitation light from the light source 420illuminates the emissive material 440 without first illuminating aninterference reflector.

Although not shown in FIG. 5, the illumination system 400 may alsoinclude one or more TIR promoting layers positioned adjacent one or bothmajor surfaces of the emissive material 440 as is described, e.g., inreference to illumination system 10 of FIG. 1.

The optical cavities of the present disclosure may use any suitabletechnique to direct light from a light source onto an interferencereflector, emissive material, or light guide. For example, FIGS.6(A)-(C) are schematic diagrams of another embodiment of an illuminationsystem 500 that includes an optical cavity 570. The system 500 alsoincludes a light guide 512 having an output surface 514 and an inputsurface 516, and a light source 520 optically coupled to the opticalcavity 570. The system 500 further includes a first interferencereflector 530 positioned between the light source 520 and the outputsurface 514 of the light guide 512, and emissive material 540 positionedbetween the first interference reflector 530 and the output surface 514of the light guide 512. All of the design considerations andpossibilities described herein with respect to the light guide 12, thelight source 20, the first interference reflector 30, and the emissivematerial 40 of the embodiment illustrated in FIG. 1 apply equally to thelight guide 512, the light source 520, the first interference reflector530, and the emissive material 540 of the embodiment illustrated inFIGS. 6(A)-(C). The system 500 can also include a LP interferencereflector as is further described herein (e.g., second interferencereflector 150 of FIG. 2).

The optical cavity 570 includes an extended aperture (not shown)adjacent the first interference reflector 530. The light source 520 maybe optically coupled to the optical cavity 570 using any suitabletechnique. For example, in FIGS. 6(A)-(C), the optical cavity 570includes a collector 571 that collects light emitted by the light source520 and directs it into optical cavity 570. In some embodiments, thecollector 571 also collimates the emitted light. As used herein, theterm “collector” refers to a non-imaging optical device that collectslight emitted by one or more light sources and directs the collectedlight toward emissive material or an interference reflector.

In some embodiments, it may be preferred that the z-dimension 502 of theoptical cavity 570 has a minimum value such that etendue is preservedwhile also maintaining total internal reflection (TIR) at the surfacesof the optical cavity 570. Such TIR at least in part depends on therefractive index of an interior space 574 of the optical cavity 570 andthe etendue of the light source 520. If the light source 520 includes anLED die, then, in some embodiments, the LED die is assumed to emit into27π steradians, in which case the TIR angle for a refractive index of1.5 is about 42°. In such embodiments, the z-dimension 502 for a 300 μmLED die is equal to (300 μm)/sin(48°)=400 μm. If the z-dimension of thelight guide 512 is 1000 μm, then the angle of incidence of the light onthe first interference reflector 530 is equal to sin⁻¹((300 μm)/(1000μm))=17.5°.

The formula for band-edge shift with angle for a multilayer film isλ=λ(0)cos(Θ)where Θ is the angle in the medium. The reflective band-edge shifts downby about 4%. Therefore, the blue band-edge selection for the firstinterference reflector 530 can be approximately 4% higher than one wouldchoose for normal incidence.

The optical cavity 570 also includes an interior space 574. The interiorspace 574 includes one or more facets 576. Each facet 576 has a facetangle 578 that is selected such that the facets 576 direct excitationlight toward the first interference reflector 530 at a substantiallynormal angle to a major surface of the first interference reflector 530.Each facet 576 has a reflective face 577 that reflects light from lightsource 520. Any suitable material or materials may be used to formfacets 576.

If the reflective surface 577 of facet 576 includes a multilayer opticalfilm, then the minimum x-dimension 504 of the optical cavity 570, whichinsures that there is little or no leakage through the facet 576 dependsupon the facet angle 578. For example, if the facet angle 578 is 45°,then some light may exceed the TIR angle at the surface 577 if thespread of light exceeds ±3°. A facet angle 578 of 45° out-couples raysat substantially normal incidence to the first interference reflector530. However, in some embodiments, it may not be necessary to illuminatethe first interference reflector 530 at perfectly normal incidence. Forexample, 10° or 20° incidence can be sufficient to ensure thatsubstantially all of the light from the light source 520 transmitsthrough to the emissive material 540. If the light spread ΔΘ is ±3° inthe optical cavity 570, then the x-dimension equals 5700 μm or 5.7 mm.Table 1 includes x-dimensions 504 for the optical cavity 570 givenvarious light spread (ΔΘ) values. TABLE 1 Optical Cavity LED x-dimensionΔΘ x-dimension 300 μm  ±3° 5.7 mm 300 μm  ±5° 3.4 mm 300 μm ±10° 1.7 mm300 μm ±15° 1.2 mm

Although the optical cavity 570 is positioned adjacent the input edge516 of the light guide 512, the optical cavity 570 first interferencereflector 530, and emissive material 540 may positioned in any suitablelocation relative to the light guide 512. For example, in someembodiments, the optical cavity 570, first interference reflector 530,and emissive material 540 may be positioned adjacent a major surface ofthe light guide 512 as is further described herein.

Some handheld light guides (e.g., light guides used in displays forhandheld electronic devices) are about 1 mm in thickness. The slim 1 mmdimension can increase the complexity of converting and assembling thefirst interference reflector 530 and the emissive material 540. If thethickness of the light guide 512 is less than 1 mm, then the embodimentschematically illustrated in FIG. 6(D) may be more useful. In FIG. 6(D),the optical cavity 570 d is adjacent a sloped input surface 516 d, whichmay allow for a larger first interference reflector 530 d. The wedgeformed by the light guide 512 d provides a zone for light to expand andmatch the numerical aperture (NA) of the light guide 512 d. An optionalsecond interference reflector 550 d that substantially transmits lightemitted by the emissive material 540 d and substantially reflects lightemitted by the light source 520 d may be positioned on the outputsurface 514 d and/or end of the light guide 512 d opposite the inputsurface 516 d to help prevent light that is not converted by theemissive material 540 d from leaving the light guide 512 d.

FIG. 6(E) schematically illustrates another embodiment of anillumination system 500 e where the optical cavity 570 e is positionedadjacent a bottom surface 518 e of light guide 512 e. Such a design mayallow for larger first interference reflectors 530 e and emissivematerial 540 e areas for smaller light guides 512 e. The system 500 ealso includes a second interference reflector 550 e positioned on theoutput surface 514 e and an end of the light guide 512 e to preventlight that is not converted by the emissive material 540 e from leavingthe light guide 512 e.

Although FIGS. 6(A)-(E) include systems having one light source, someembodiments can include two or more light sources. For example, FIG. 7schematically illustrates an illumination system 600 including fourlight sources 620, each optically coupled to optical cavities 670.Optical cavities 670 may include any suitable optical cavity describedherein, e.g., optical cavity 570 of FIGS. 6(A)-(C). Each optical cavity670 is positioned adjacent an input surface 616 a and 616 b of the lightguide 612. The illumination system 600 may include any suitable systemas described herein, e.g., illumination system 100 of FIG. 2. Althoughsystem 600 includes optical cavities 670 adjacent two input surfaces616(a) and 616(b) of light guide 612, the system 600 may include anysuitable number of optical cavities positioned in any suitable locationsuch that additional light sources may be provided.

As is also further described herein, any of the disclosed interferencereflectors may be curved to aid in maintaining a substantially normalangle of incidence of light emitted by a point source onto theinterference reflectors. For example, FIGS. 8(A)-(B) schematicallyillustrate one embodiment of an illumination system 700 having a curvedfirst interference reflector 730. The illumination system 700 is similarto the illumination system 10 of FIG. 1. The system 700 includes a lightguide 712 having an output surface 714 and an input surface 716, and oneor more light sources 720. The system 700 further includes a firstinterference reflector 730 positioned between the one or more lightsources 720 and the output surface 714, and emissive material 740positioned between the first interference reflector 730 and the outputsurface 714 of the light guide 712. All of the design considerations andpossibilities described herein with respect to the light guide 12, thelight source 20, the first interference reflector 30, and the emissivematerial 40 of the embodiment illustrated in FIG. 1 apply equally to thelight guide 712, each of the one or more light sources 720, the firstinterference reflector 730, and the emissive material 740 of theembodiment illustrated in FIGS. 8(A)-(B). The system 700 may alsoinclude optional second interference reflector 750 positioned betweenthe emissive material 740 and the output surface 714 of the light guide712 as is further described herein.

In the embodiment illustrated in FIGS. 8(A)-(B), the one or more lightsources 720 may be mounted on interconnect assembly 724. Any suitableinterconnect assembly may be used, e.g., those assemblies described inco-owned and copending U.S. Patent Application Publication No.2005/0116235 (Schultz et al.).

The system 700 further includes one or more optical cavities 770positioned within the light guide 712. In the embodiment illustrated inFIGS. 8(A)-(B), each light source 720 is associated with an opticalcavity 770. The one or more optical cavities 770 may take any suitableshape, e.g., cylindrical, hemispherical, etc. In the embodimentillustrated in FIGS. 8(A)-(B), each optical cavity 770 is hemispheric inshape. All of the one or more optical cavities 770 may take the sameshape. Alternatively, one or more optical cavities 770 may takedifferent shapes. Further, each optical cavity 770 may be of anysuitable size.

The optical cavities may be bounded by reflective surface 772. Anysuitable material or materials may be used to form reflective surface772. It may be preferred that the reflective surface 772 include abroadband interference reflector as described, e.g., in U.S. Pat. No.5,882,774 (Jonza et al.).

In the embodiment illustrated in FIGS. 8(A)-(B), the one or more opticalcavities 770 are positioned in an interior space 717 of the light guide712. The one or more optical cavities 770 may be formed using anysuitable technique. For example, the one or more optical cavities 770may be formed as indentations in the input surface 716 of the lightguide 712. Any suitable number of optical cavities 770 may be includedin the illumination system 700. Further, although FIGS. 8(A)-(B)illustrate optical cavities 770 on one edge of light guide 712, thesystem 700 may include optical cavities 770 on two or more sides of thelight guide 712 or on one or more major surfaces of the light guide 712.

In some embodiments, each light source 720 may be positioned proximate acenter of curvature of each optical cavity 770. By placing the lightsource 720 proximate the center of curvature of each optical cavity 770,light emitted by the light source 720 may illuminate the firstinterference reflector 730 substantially normal to a major surface ofthe first interference reflector 730, thereby eliminating some bandedgeshift. In other words, spacing the first interference reflector 730 awayfrom the light source 720 and curving it in towards the light source 720may help reduce the range of incident angles of light impinging on thefirst interference reflector 730, thereby reducing the leakage of lightthrough the first interference reflector 730 caused by the blue-shifteffect as described herein.

In general, light having a first optical characteristic is emitted bythe light source 720 and is substantially transmitted by the firstinterference reflector 730. The transmitted light illuminates theemissive material 740, causing the emissive material 740 to emit lighthaving a second optical characteristic. Any light emitted by theemissive material 740 toward the light source 720 is substantiallyreflected by the first interference reflector 730. Further, any lightnot transmitted by the first interference reflector 730 is substantiallyreflected by the reflective surface 772 and directed back toward thefirst interference reflector 730. The light emitted by the emissivematerial 740 is then directed by the light guide 712 through the outputsurface 714 to a desired location. If an optional second interferencereflector 750 is included between the emissive material 740 and theoutput surface 714, it may be preferred that the second interferencereflector 750 substantially transmits light having the second opticalcharacteristic and substantially reflects light having the first opticalcharacteristic. In such an exemplary embodiment, the light emitted bythe emissive material 740 would be substantially transmitted by thesecond interference reflector 750 and directed by the light guide 712through the output surface 714 to a desired location. Light emitted bythe light source 720 that passes through the emissive material 740without being absorbed is substantially reflected by the secondinterference reflector 750 back toward the emissive material 740.

As previously described herein, some light sources of the presentdisclosure emit excitation light in a pattern having a solid angle of 2πsteradians or greater. In some embodiments, a collector may be used tocollect light emitted by a light source and collimate the collectedlight such that the light is directed toward an interference reflectoror emissive material at substantially normal angles.

FIGS. 9(A)-(B) schematically illustrate one embodiment of anillumination system 800 having one or more collectors 880. Theillumination system 800 includes a light guide 812 having an outputsurface 814 and an input surface 816, and a light source 820. In theembodiment illustrated in FIGS. 9(A)-(B), the light source 820 includesone or more LEDs 822 optionally mounted on an interconnect assembly 824as is further described herein. The system 800 also includes a firstinterference reflector 830 positioned between the light source 820 andthe output surface 814, and emissive material 840 positioned between thefirst interference reflector 830 and the output surface 814 of the lightguide 812. All of the design considerations and possibilities describedherein with respect to the light guide 12, the light source 20, thefirst interference reflector 30, and the emissive material 40 of theembodiment illustrated in FIG. 1 apply equally to the light guide 812,the light source 820, the first interference reflector 830, and theemissive material 840 of the embodiment illustrated in FIGS. 9(A)-(B).Although not shown, the system 800 may also include an optional LPinterference reflector between the emissive material 840 and the outputsurface 814 as previously described herein.

One difference between the illumination system 800 of FIGS. 9(A)-(B) andthe illumination system 10 of FIG. 1 is that each LED 822 is associatedwith a collector 880. Each collector 880 forms an optical cavity 882that directs light emitted by the LED 822 toward the first interferencereflector 830. Each collector 880 may take any suitable shape, e.g.,spherical, parabolic, or elliptical. It may be preferred that eachcollector 880 take a shape that allows for collimation of the lightemitted by the light source 820. Further, it may be preferred that eachcollector 880 be shaped such that it collects the light emitted by theLED 822 and directs the light toward the first interference reflector830 such that the excitation light is incident upon the firstinterference reflector 830 at an angle that is substantially normal to amajor surface of the first interference reflector 830. The collectors880 can reduce the angular spread of light impinging on the firstinterference reflector 830, thus reducing the blue-shift of thereflection band as is further described herein. Each collector 880 maybe in the form of simple conical sections with flat sidewalls, or thesidewalls can take on a more complex curved shape as is known to enhancecollimation or focusing action depending on the direction of lighttravel. It may be preferred that the sidewalls of the collectors 880 arereflective and the two ends are not. It may also be preferred that thecollector's sidewalls include a broadband interference reflector as isfurther described herein. Each collector 880 may be positioned in anysuitable relationship to the first interference reflector 830. Forexample, each collector 880 may be spaced apart from the firstinterference reflector 830. Alternatively, one or more collectors 880may be in contact with the first interference reflector 830.

Although the system 800 is illustrated as having a light source 820positioned adjacent one input surface 816 of light guide 812, the system800 can include two or more light sources positioned adjacent two ormore input surfaces of the light guide 812.

Any suitable device or technique may be used with the embodiments of thepresent disclosure to direct light from a light source toward aninterference reflector such that the light is incident upon theinterference reflector at substantially normal angles. For example,FIGS. 10(A)-(B) schematically illustrate one embodiment of anillumination system 900 that includes an optical cavity 970 havingcollectors 980 formed in the optical cavity 970. The system 900 includeslight source 920. The light source 920 includes one or more LEDs 922. Inthis embodiment, the light source 920 is positioned adjacent an inputsurface 916 of light guide 912. The system 900 further includes a firstinterference reflector 930 positioned between the light source 920 andthe output surface 914, and emissive material 940 positioned between thefirst interference reflector 930 and the output surface 914. All of thedesign considerations and possibilities in regard to the light guide 12,the light source 20, the first interference reflector 30, and theemissive material 40 of the embodiment illustrated in FIG. 1 applyequally to the light guide 912, the light source 920, the firstinterference reflector 930, and the emissive material 940 of theembodiment illustrated in FIGS. 10(A)-(B). The system 900 may alsoinclude a LP interference reflector, e.g., second interference reflector150 of FIG. 2.

The optical cavity 970 is positioned to direct light emitted by thelight source 920 into the light guide 912. The optical cavity 970includes collectors 980 formed in the optical cavity 970. Each LED 922has a corresponding collector 980. In some embodiments, two or more LEDs922 may be positioned within a single collector 980. The collectors 980may each take any suitable shape, e.g., hemispherical, parabolical, orcylindrical. In FIGS. 10(A)-(B), the collectors 980 are shaped as atwo-dimensional conic sections. It may be preferred that each collector980 is shaped such that light emitted by each LED 922 illuminates thefirst interference reflector 930 at substantially normal angles to amajor surface of the first interference reflector 930. The collectors980 collect light emitted by the LEDs 922 and direct the collected lightsuch that it illuminates the first interference reflector 930. Further,it may be preferred that one or more LEDs 922 be positioned proximate afocus of one or more collectors 980.

Any suitable technique may be used to form optical cavity 970 andcollectors 980. In some embodiments, the LEDs 922 may be potted with aflat slab encapsulant and index-matched to the optical cavity 970.Further, the first interference reflector 930 and the emissive material940 may be optically coupled to the optical cavity 970 using anysuitable material or materials, e.g., optical adhesives, etc. It may bepreferred that a TIR promoting layer may be positioned between theoptical cavity 970 and the light guide 912 for better NA match of thelight emitted by the emissive material 940 into the light guide 912.

In some embodiments, the LEDs 922 may be mounted on an interconnectassembly 924. Any suitable interconnect assembly may be used, e.g.,those interconnect assemblies described in co-owned and copending U.S.Patent Application Publication No. 2005/0116235 (Schultz et al.). In anexemplary embodiment, the optical cavity 970 may be formed oninterconnect assembly 924 using any suitable technique.

The collectors 980 may include a reflective inner surface such thatlight emitted by each LED 922 is reflected toward the first interferencereflector 930. It may be preferred that one or more collectors 980include a broadband interference reflector positioned in the collector980 to reflect light toward the first interference reflector 930.

As previously described herein, the interference reflectors and emissivematerial of the present disclosure may be positioned in any suitablerelationship to the light guide. For example, the first interferencereflector 30 and emissive material 40 of illumination system 10 of FIG.1 are positioned adjacent the input surface 16 of the light guide 12. Insome embodiments, conversion of light can occur adjacent an outputsurface of a light guide. In other words, light from a light source maybe directed by a light guide through an output surface of the lightguide and subsequently converted by emissive material positioned on oradjacent the output surface of the light guide. Depending upon the typesof light sources and interference reflectors selected, positioning theemissive material and interference reflectors a distance from the lightsource may prevent damage to the emissive material and/or theinterference reflectors.

For example, polymeric interference reflectors can be degraded byoverheating which can cause material creep thereby changing the layerthickness values and therefore the optical characteristics of the light(e.g., wavelength) that the reflector reflects. In the worst case,overheating can cause the polymer materials to melt, resulting in rapidflow of material and change in optical characteristic selection as wellas inducing non-uniformities in the filter.

Degradation of polymer materials can also be induced by short wavelength(actinic) radiation such as blue, violet, or UV radiation, depending onthe polymer material. The rate of degradation is dependent both on theactinic light flux and on the temperature of the polymer. Both thetemperature and the flux will, in general, decrease with increasingdistance from the light source. Thus it is advantageous in cases of highbrightness light sources, particularly UV emitting light sources, toplace polymeric interference reflectors as far from the light source asthe design can allow.

FIG. 11 schematically illustrates one embodiment of an illuminationsystem 1000 including a light guide 1012 having an output surface 1014and an input surface 1016, and a light source 1020. The light source1020 emits light having a first optical characteristic. The system 1000also includes emissive material 1040 positioned to receive light fromthe output surface 1014 of the light guide 1012, and a firstinterference reflector 1030 positioned between the emissive material1040 and the output surface 1014 of the light guide 1012. The emissivematerial 1040 emits light having a second optical characteristic whenilluminated with light having the first optical characteristic. Thefirst interference reflector 1030 substantially transmits light havingthe first optical characteristic and substantially reflects light havingthe second optical characteristic. All of the design considerations andpossibilities described herein with respect to the light guide 12, thelight source 20, the first interference reflector 30, and the emissivematerial 40 of the embodiment illustrated in FIG. 1 apply equally to thelight guide 1012, the light source 1020, the first interferencereflector 1030, and the emissive material 1040 of the embodimentillustrated in FIG. 11. The system 1000 may also include a secondinterference reflector 1050 positioned such that the emissive material1040 is between the second interference reflector 1050 and the firstinterference reflector 1030. Any suitable interference reflectordescribed herein may be utilized for the second interference reflector1050 (e.g., second interference reflector 150 of FIG. 2). The secondinterference reflector 1050 may help to prevent some or all of the lightemitted by the light source 1020 from reaching a viewer facing theoutput surface 1014 of the light guide 1012. The second interferencereflector 1050 may be positioned in any suitable location. In someembodiments, the second interference reflector 1050 may be positioned onand in contact with the emissive material 1040.

The first interference reflector 1030 may be positioned adjacent theoutput surface 1014, on the output surface 1014, on the emissivematerial 1040, or in any other suitable location. In one exemplaryembodiment, the first interference reflector 1030 may be on and incontact with both the emissive material 1040 and the output surface 1014of the light guide 1012. In some embodiments, system 1000 may alsoinclude one or more TIR promoting layers between the output surface 1014and the first interference reflector 1030, and/or an extraction deviceor devices on output surface 1014 to extract light from the light guide1012. Any suitable extraction device may be utilized.

In some embodiments, an extraction device or devices may be includedadjacent a bottom surface 1018 of the light guide 1012 to direct atleast a portion of light within the light guide 1012 though the outputsurface 1014. Any suitable extraction device or devices may be utilized.

In some embodiments, the illumination system 1000 may include a TIRpromoting layer in contact with the emissive material 1040 between thefirst interference reflector 1030 and the emissive material 1040. It maybe preferred that the TIR promoting layer include an index of refractionat the wavelength of light emitted by the light source 1020 that is lessthan the index of refraction of the emissive material 1040. Any suitablematerial or materials may be used for the TIR promoting layer. The TIRpromoting layer may include an air gap; alternatively, the TIR promotinglayer may include a microstructured layer.

A second TIR promoting layer may be positioned in contact with theemissive material 1040 between the emissive material 1040 and theoptional second interference reflector 1050. It may be preferred thatthe second TIR promoting layer include an index of refraction at thewavelength of light emitted by the light source 1020 that is less thanthe index of refraction of the emissive material 1040.

Although not shown, the system 1000 may include one or more opticalelements positioned to receive light emitted by the emissive material1040. Alternatively, the one or more optical elements may be positionedbetween the output surface 1014 and the first interference reflector1030, and/or between the light source 1020 and the output surface 1014of the light guide 1014. If a second interference reflector 1050 isincluded, then one or more optical elements may be positioned betweenthe emissive material 1040 and the second interference reflector 1050,and/or such that the second interference reflector is between theemissive material 1040 and the one or more optical elements. The one ormore optical elements may include any suitable optical element as isfurther described herein.

In general, light having a first optical characteristic is emitted bythe light source 1020, at least a portion of which enters the lightguide 1012 and is directed through the output surface 1014. At least aportion of the light from the light guide 1012 illuminates the firstinterference reflector 1030 and is substantially transmitted. At least aportion of the transmitted light illuminates the emissive material 1040,thereby causing the emissive material 1040 to emit light having a secondoptical characteristic. Light emitted by the emissive material 1040 canthen be directed to a desired location using any suitable technique. Anylight emitted by the emissive material 1040 toward the firstinterference reflector 1030 is substantially reflected back toward theemissive material. If a second interference reflector 1050 is includedin system 1000, then light emitted by the emissive material 1040 thatilluminates the second interference reflector 1050 is substantiallytransmitted and directed to a desired location. Any light emitted by thelight source 1020 that illuminates the second interference reflector1050 is substantially reflected back toward the emissive material 1040where it may be converted to light having the second opticalcharacteristic.

Alternatively, some embodiments of illumination systems of the presentdisclosure may include a LP interference reflector and no SPinterference reflector. For example, FIG. 17 schematically illustratesan embodiment of an illumination system 1600 that includes a lightsource 1620 and a light guide 1612 having an output surface 1614 and aninput surface 1616. The light source 1620 emits light having a firstoptical characteristic. The system 1600 also includes emissive material1640 positioned to receive light from the output surface 1614 and aninterference reflector 1650 positioned such that the emissive material1640 is between the output surface 1614 and the interference reflector1650. The emissive material 1640 emits light having a second opticalcharacteristic when illuminated with light having the first opticalcharacteristic. In this exemplary embodiment, the interference reflector1650 substantially transmits light having the second opticalcharacteristic and substantially reflects light having the first opticalcharacteristic. All of the design considerations and possibilitiesdescribed herein with respect to the light guide 1012, the light source1020, the emissive material 1040, and the second interference reflector1050 of the embodiment illustrated in FIG. 11 apply equally to the lightguide 1612, the light source 1620, the emissive material 1640, and theinterference reflector 1650 of the embodiment illustrated in FIG. 17.The illumination system 1600 may also include other elements asdescribed in reference to illumination system 1000 of FIG. 11, e.g., oneor more optical elements, TIR promoting layers, etc.

In general, light having a first optical characteristic is emitted bythe light source 1620, at least a portion of which enters the lightguide 1612 and is directed through the output surface 1614. At least aportion of the light from the light guide 1612 illuminates the emissivematerial 1640, thereby causing the emissive material 1640 to emit lighthaving a second optical characteristic. At least a portion of the lightemitted by the emissive material 1640 is substantially transmitted bythe interference reflector 1650 and directed to a desired location usingany suitable technique. Any light emitted by the light source 1020 thatilluminates the interference reflector 1050 is substantially reflectedback toward the emissive material 1040 where it may be converted tolight having the second optical characteristic.

FIG. 12 schematically illustrates another embodiment of an illuminationsystem 1100. The system 1100 includes a light guide 1112 having anoutput surface 1114 and an input surface 1116, and a light source 1120.The light source 1120 emits light having a first optical characteristic.The system 1100 further includes a first interference reflector 1130positioned adjacent the output surface 1114. The first interferencereflector 1130 substantially transmits light having the first opticalcharacteristic and substantially reflects light having a second opticalcharacteristic. The first interference reflector 1130 includesindentations 1134 formed in a first major surface 1132 of the firstinterference reflector 1130. The system 1100 also includes emissivematerial 1140 positioned to receive excitation light from the outputsurface 1114 of the light guide 1112. The system 1100 may also includean optional LP interference reflector (not shown) positioned such thatthe emissive material 1140 is between the LP interference reflector andthe first interference reflector 1130. All of the design considerationand possibilities in regard to the light guide 112, the light source120, the first interference reflector 130, the emissive material 140,and the second interference reflector 150 of the embodiment illustratedin FIG. 2 apply equally to the light guide 1112, the light source 1120,the first interference reflector 1130, the emissive material 1140, andthe optional LP interference reflector of the embodiment of FIG. 12.

The emissive material 1140 includes dots 1142 that are positioned withinindentations 1134 formed in the major surface 1132 of the interferencereflector 1130. Each phosphor dot can have any suitable size. Forexample, each dot can have an area in plan view of less than 10000 μm²or from 500 to 10000 μm². In an illustrative embodiment, the dots caneach be formed from emissive material that emits light having the secondoptical characteristic when illuminated with light having the firstoptical characteristic. In some embodiments, the emissive material 1140includes one or more dots that emit one or more emitted wavelengths ofvisible light, e.g., a dot emitting red, a dot emitting blue, and a dotemitting green. For example, phosphor dot 1142R may emit red light whenilluminated with light from the light source 1120, phosphor dot 1142Gmay emit green light, and phosphor dot 1142B may emit blue light.

The dots 1142 may be arranged and configured in any uniform ornon-uniform manner as desired. For example, the emissive material 1140can be a number of dots with a non-uniform density gradient along asurface or an area. The dots can have any regular or irregular shape andneed not be round in plan view.

In general, structured phosphor layers, e.g., dots, can be configured inseveral ways to provide benefits in performance as described herein.When various types of emissive materials are used (e.g., red emitters,green emitters, etc.), light emitted from shorter wavelength emissivematerials can be re-absorbed by other emissive materials. Patternsincluding isolated dots, lines, or isolated regions of each type canreduce the amount of reabsorption.

Any suitable technique may be used to provide indentations 1134 in themajor surface 1132 of interference reflector 1030, e.g., thermoforming,embossing, knurling, laser marking or ablating, abrading, cast and cure,etc. Alternatively, the first interference reflector 1030 may bethermoformed to provide reflective wells or pockets within whichemissive material 1140 may be placed. The indentations 1134 may beformed in any pattern. Each indentation 1134 may have any suitabledepth. It may be preferred that each indentation 1134 be relativelyshallow such that the first interference reflector 1130 is notexcessively thinned. Such thinning may cause a large wavelength shiftdue to thickness or angle effects.

Although FIG. 12 illustrates the emissive material 1140 as includingdots 1142, the emissive material 1140 may be formed in any suitableshape and/or pattern, e.g., lines, discrete shapes, or half-tonespatterned in graded density and/or size.

The light guides of the present disclosure can take any suitable shape.For example, FIG. 13 is a schematic diagram of another embodiment of anillumination system 1200. The system 1200 is similar in many respects tothe illumination system 1000 of FIG. 11. The illumination system 1200includes a light guide 1212 having an output surface 1214 and an inputsurface 1216, and a light source 1220. The system 1200 also includesemissive material 1240 positioned to receive light emitted by the lightsource 1220 from the output surface 1214 of the light guide 1212, and afirst interference reflector 1230 positioned between the emissivematerial 1240 and the output surface 1214. All of the designconsiderations and possibilities described herein with respect to thelight guide 1012, the light source 1020, the first interferencereflector 1030, and the emissive material 1040 of the embodimentillustrated in FIG. 11 apply equally to the light guide 1212, the lightsource 1220, the first interference reflector 1230, and the emissivematerial 1240 of the embodiment illustrated in FIG. 13. The system 1200may also include a LP interference reflector (e.g., second interferencereflector 1050 of FIG. 11) positioned such that the emissive material1240 is between the first interference reflector 1230 and the LPinterference reflector.

The system 1200 also includes an optical cavity 1270 optically coupledto the light source 1220 that directs excitation light from the lightsource 1220 into the light guide 1212. Any suitable optical cavity 1270may be used, e.g., optical cavity 470 of the embodiment illustrated inFIG. 5.

The light guide 1212 also includes a reflective bottom surface 1218 thatforms an angle with the output surface 1214 such that the light guide1212 takes a wedge-like shape that tapers distal from the input surface1216. The reflective bottom surface 1218 may include any suitablereflective material or materials. It may be preferred that thereflective bottom surface 1218 include a broadband interferencereflector 1290 as described, e.g., in U.S. Pat. No. 5,882,774 (Jonza etal.). The broadband interference reflector 1290 may be in contact withor spaced from bottom surface 1218.

In some embodiments, a TIR promoting layer may be positioned between theoutput surface 1214 and the first interference reflector 1230, and/orbetween the first interference reflector 1230 and the emissive material1240 as is previously described herein.

The use of a wedged light guide 1212 may provide substantially normalincidence of the light emitted by the light source 1220 on the firstinterference reflector 1230, thereby allowing transmission ofsubstantially all of the light on the first pass towards the firstinterference reflector 1230. In embodiments where there is a TIRpromoting layer between the output surface 1214 and the firstinterference reflector 1230, light directed toward the output surface1214 from within the light guide 1212 at oblique angles may be directedback into the light guide 1212 by the TIR promoting layer. Suchredirected light may then be reflected by the reflective bottom surface1218 and directed through the output surface 1214 at substantiallynormal incidence to the output surface 1214. Some light within the lightguide 1212 may be directed through the input surface 1216 toward thelight source 1220. Such light may be collected by optical cavity 1270and redirected into the light guide 1212 through the input surface 1216.

In general, the light source 1220 emits light having a first opticalcharacteristic that is directed into the light guide 1212 by the opticalcavity 1270. At least a portion of light is directed by the light guide1212 and/or the reflective bottom surface 1218 of the light guide 1212through the output surface 1214 such that it illuminates the firstinterference reflector 1230. The first interference reflector 1230substantially transmits light having the first optical characteristiconto the emissive material 1240. When illuminated with light having thefirst optical characteristic, the emissive material 1240 emits lighthaving a second optical characteristic. Some light may be emitted by theemissive material 1240 back toward the output surface 1214 of the lightguide 1212. The first interference reflector 1230 may substantiallyreflect such light back away from the output surface 1214.

The illumination systems of the present disclosure may include anysuitable type of light guide or guides. For example, FIG. 14schematically illustrates one embodiment of an illumination system 1300that includes light guides 1312 each having an input surface 1316 and anoutput surface 1314, and a light source 1320. The light guides 1312 areoptically coupled to the light source 1320. The light source 1320 emitslight having a first optical characteristic. The system further includesemissive material 1340 positioned to receive light from at least onelight guide 1312, and a first interference reflector 1330 positionedbetween the emissive material 1340 and the output surfaces 1314 of thelight guides 1312. The first interference reflector 1330 substantiallytransmits light having the first optical characteristic andsubstantially reflects light having a second optical characteristic. Theemissive material 1340 emits light having the second opticalcharacteristic when illuminated with light having the first opticalcharacteristic. The system 1300 may also include an optional secondinterference reflector 1350 positioned such that the emissive material1340 is between the second interference reflector 1350 and the firstinterference reflector 1330. All of the design considerations andpossibilities in regard to the light guide 112, the light source 120,the first interference reflector 130, the emissive material 140, and thesecond interference reflector 150 of the embodiment illustrated in FIG.2 apply equally to the light guides 1312, the light source 1320, thefirst interference reflector 1330, the emissive material 1340, and theoptional second interference reflector 1350 of the embodimentillustrated in FIG. 14.

In some embodiments, the light guides 1312 may include one or moreoptical fibers 1313. The optical fibers 1313 may include any suitabletype of optical fibers, e.g., large-core polymer clad silica fibers(such as those marketed under the trade designation TECS™, availablefrom 3M Company, St. Paul, Minn.), glass fibers, plastic core opticalfibers, etc.

The optical fibers 1313 are optically coupled to the light source 1320.As previously described herein, the light source 1320 may include anysuitable type of light source or sources. In some embodiments, the lightsource 1320 may include discrete LED dies or chips disposed in an arraypattern. Further, in some embodiments, the illumination system 1300 caninclude one optical fiber 1313 for each light source 1320.

Any suitable technique may be used to couple light emitted by the lightsource 1320 into the light guides 1312. For example, the illuminationsystem 1300 may include one or more collectors that can convertisotropic emission from a corresponding LED die into a beam that willmeet the acceptance angle criteria of a corresponding light-receivinglight guide as described in the following co-owned and copending patentapplications: U.S. Pat. No. 7,163,327 (Henson et al.); U.S. PatentApplication Publication No. 2005/0117366 (Simbal); U.S. PatentApplication Publication No. 2005/0116635 (Watson et al.); U.S. PatentApplication Publication No. 2005/0116235 (Schultz et al.); U.S. PatentApplication Publication No. 2005/0140270 (Henson et al.); U.S. PatentApplication Publication No. 2005/0116176 (Aguirre et al.); and U.S.Patent Application Publication No. 2005/0134527 (Ouderkirk et al.).

The emissive material 1340 as well as the first interference reflector1330 and/or second interference reflector 1350 may take any suitableshape as is further described herein. In some embodiments, the emissivematerial 1340 and one or both interference reflectors 1330 and 1350 maybe in the form of a continuous layer or layers. In other embodiments,the emissive material 1340 and one or both of the interferencereflectors 1330 and 1350 may be curved. Further, in some embodiments,the emissive material 1340 and one or both interference reflectors 1330and 1350 may be non-continuous segments that are formed on and incontact with one or more output surfaces 1314 of the light guides 1312.

The emissive material 1340 may be positioned in any suitablerelationship to the output surfaces 1314 of the light guides 1312. Insome embodiments, the emissive material 1340 may be spaced apart fromthe output surfaces 1314. In other embodiments, the emissive material1340 may be positioned on one or both of the first interferencereflector 1330 and the optional second interference reflector 1350. Inother embodiments, a TIR promoting layer or layers may be positioned onthe emissive material 1340 between the emissive material 1340 and thefirst interference reflector 1330, between the emissive material 1340and the optional second interference reflector 1350, or on both sides ofthe emissive material 1340 as is further described herein. See also U.S.Pat. No. 7,157,839 (Ouderkirk et al.).

The first interference reflector 1330 may be positioned in any suitablelocation relative to the output surfaces 1314 and the emissive material1340, e.g., spaced apart from the output surfaces 1314, spaced apartfrom the emissive material 1340, on the output surfaces 1314, on theemissive material 1340, on both the output surfaces 1314 and theemissive material 1340, etc. In some embodiments, a TIR promoting layeror layers may be included between the output ends 1314 and the firstinterference reflector 1330. Further, in some embodiments, the outputsurfaces 1314 of the light guides 1312 and the first interferencereflector 1330 may be index-matched using any suitable technique ormaterials, e.g., using index matching fluids, gels., adhesives, pressuresensitive adhesives, UV cured adhesives, or cements.

The illumination system 1300 may also include one or more opticalelements 1360. The one or more optical elements 1360 may be positionedto receive light from the emissive material 1340, between the emissivematerial 1340 and the first interference reflector 1330, and/or betweenthe output surfaces 1314 of the light guides 1312 and the firstinterference reflector 1330. The one or more optical elements 1360 mayinclude collimating optics for directing light within a predeterminedangle toward a display or other device. For example, the one or moreoptical elements 1360 may include brightness enhancement films, turningfilms, lenses, diffusers, gain diffusers, contrast-enhancing materials,reflective elements, etc. In some embodiments, the one or more opticalelements 1360 may include a walk-off plate or crystal to provide a moreuniform light distribution. Walk-off plates or crystals include layersthat separate a ray of light into two rays that are displaced from eachother, where such displacement results from the two polarization statesof a light ray that each encounter different degree of refraction whenimpinging upon the walk-off crystal. Typical walk-off plates are madefrom a material that has different refractive indices for differentpolarizations of light (i.e., birefringence). Typically, the highrefractive index direction is skewed from at least one of the in-planeaxes of the plate.

In some embodiments, the one or more optical elements 1360 may include areflective polarizer that allows light of a preferred polarization to beemitted by the system 1300, while reflecting the other polarization. Anysuitable reflective polarizer may be utilized, e.g., cholestericreflective polarizers, cholesteric reflective polarizers with a ¼ waveretarder, wire grid polarizers, and a variety of reflective polarizersavailable from 3M Company, including DBEF (i.e., a specularly reflectivepolarizer), DRPF (i.e., a diffusely reflective polarizer). Lightreflected by the reflective polarizer 1360 can be depolarized by theemissive material 1340, and/or the interference reflectors 1330 and 1350and recycled such that light of the selected polarization can be emittedwith greater efficiency.

In general, light from the light source 1320 illuminates the inputsurfaces 1316 of the light guides 1312 and is directed by the lightguides 1312 through the output surfaces 1314 where at least a portion ofsuch light illuminates the first interference reflector 1330. The firstinterference reflector 1330 substantially transmits the light from thelight source 1320 such that at least a portion of the light illuminatesthe emissive material 1340.

The emissive material 1340 emits light having the second opticalcharacteristic when illuminated with light having the first opticalcharacteristic. For example, the emissive material 1340 may be selectedsuch that it emits visible light when illuminated with UV or blue lightfrom the light source 1320. At least a portion of the light emitted bythe emissive material 1340 illuminates the optional second interferencereflector 1350, which substantially transmits such light. Any light fromthe light source 1320 that is not converted by the emissive material1340 is substantially reflected by the optional second interferencereflector 1350 back toward the emissive material 1340. Further, anylight emitted by the emissive material 1340 that illuminates the firstinterference reflector 1330 is substantially reflected.

As previously mentioned herein, any suitable technique may be used tocouple light from the light source 1320 into the light guides 1312. Forexample, FIG. 15 schematically illustrates another embodiment of anillumination system 1400 that includes light guides 1412 includingoptical fibers 1413. See, e.g., co-owned and copending U.S. Pat. No.7,163,327 (Henson et al.). The system 1400 includes a light source 1420,emissive material 1440 positioned to receive light from the light source1420, and a first interference reflector 1430 positioned between thelight guides 1412 and the emissive material 1440. All of the designconsiderations and possibilities in regard to the light guide 1312, thelight source 1320, the first interference reflector 1330, and theemissive material 1340 of the embodiment illustrated in FIG. 14 applyequally to the light guide 1412, the light source 1420, the firstinterference reflector 1430, and the emissive material 1440 of theembodiment illustrated in FIG. 15. The system 1400 may also include asecond interference reflector (not shown) as is further describedherein.

Light source 1420 includes an array 1422 of LED dies 1424 that arepositioned in optical alignment with an array of optical elements 1428,which can include passive optical elements, such as focusing lenses 1429or optical concentrating elements, such as reflectors. The array ofoptical elements 1428 are in turn optically aligned to an array ofoptical fibers 1413. The array of optical fibers 1413 can beconnectorized, where the connectorization can include a connector 1417to support and/or house input surfaces 1416 of fibers 1413. Theconnectorization can also include a connector 1415 to support and/orhouse output surfaces 1414 of fibers 1413. Any suitable connector orconnectors may be used at either the input surfaces 1416 or outputsurfaces 1414 of the optical fibers 1413, e.g., those described in U.S.patent Ser. No. 10/726,222 (Henson et al.). As would be apparent to oneof ordinary skill in the art given the present description, the outputsurfaces 1414 of the fibers 1413 may be bundled to form a point-likesource or a shaped-array, such as a linear array, circular array,hexagonal array, or other shaped-array.

In an exemplary embodiment, the array 1422 of light source 1420 includesan array of discrete LEDs 1424, such as an array of single LED dies orchips, which are mounted individually and have independent electricalconnections for operational control (rather than an LED array where allthe LEDs are connected to each other by their common semiconductorsubstrate). LED dies can produce a symmetrical radiation pattern, makingthem desirable light sources for the present disclosure. LED dies areefficient at converting electrical energy to light and are not astemperature sensitive as most laser diodes. Therefore, LED dies mayoperate adequately with only a modest heat sink compared to many typesof laser diodes. In an exemplary embodiment, each LED die 1424 is spacedapart from its nearest neighbor(s) by at least a distance greater thanan LED die width.

In addition, LED dies can be operated at a temperature from −40° to 125°C. and can have operating lifetimes in the range of 100,000 hours, ascompared to most laser diode lifetimes around 10,000 hours or halogenautomobile headlamp lifetimes of 500-1000 hours. In an exemplaryembodiment, the LED dies 1424 can each have an output intensity of about50 Lumens or more. Discrete high-power LED dies are commerciallyavailable from companies such as Cree and Osram. In one exemplaryembodiment, an array of LED dies 1424 (manufactured by Cree), eachhaving an emitting area of about 300 μm×300 μm, can be used to provide aconcentrated (small area, high power) light source. Other light emittingsurface shapes such as rectangular or other polygonal shapes can also beutilized. In addition, in alternative embodiments, the emission layer ofthe LED dies 1424 utilized can be located on the top or bottom surface.

In an alternative embodiment, the array 1422 may be replaced with awhite vertical cavity surface emitting laser (VCSEL) array. The passiveoptical element array 1428 may be used to redirect that light emittedfrom each VCSEL into a corresponding fiber 1413.

An aspect of the illustrated embodiment of FIG. 15 is the one-to-onecorrespondence between each light source 1412, a corresponding passiveoptical element of the array of optical elements 1428 (lens, focusing,concentrating, or reflective element), and a corresponding optical fiber1413. When powered, each LED die 1424 acts as an individual light sourcethat launches light into a corresponding fiber 1413. The presentexemplary embodiment includes large-core (for example, 400 μm to 1000μm) polymer clad silica fibers (such as those marketed under the tradedesignation TECS™, available from 3M Company, St. Paul, Minn.). Othertypes of optical fibers, such as conventional or specialized glassfibers may also be utilized in accordance with the embodiments of thepresent disclosure, depending on such parameters, e.g., as the outputwavelength(s) of the LED dies 1424.

In addition, as would be apparent to one of ordinary skill given thepresent description, other waveguide types, such as planar waveguides,polymer waveguides, or the like, may also be utilized in accordance withthe present teachings.

Optical fibers 1413 may further include fiber lenses on each of theoutput surfaces 1414 of the optical fibers 1413. Similarly, the inputsurfaces 1416 of the optical fibers 1413 may each further include afiber lens. Fiber lens manufacture and implementation is described inco-owned and copending U.S. Pat. No. 6,822,190 (Smithson et al.) andU.S. Patent Application Publication No. 2005/0069256 (Jennings et al.).

The individual optical fibers 1413 are collected together to provideremote lighting at a distance from the original light sources. A furtherdescription of an LED-based lighting assembly that is implanted as abulb replacement is described in co-owned and copending U.S. PatentApplication Publication No. 2005/0140270 (Henson et al.).

In some embodiments, the LED dies 1424 may be independently controllablesuch that one or more LEDs 1424 can be selectively activated. Forexample, the system 1400 may include a controller (not shown) that is inelectrical communication with each LED 1424. The controller is operableto selectively activate one or more LEDs 1424. Any suitable controlleror controllers may be used, e.g., those described in co-owned andcopending U.S. Pat. No. 7,163,327 (Henson et al.). Such controllableoutput of the LEDs 1424 may be used in various types of applications,e.g., steerable headlamps for motor vehicles, pixilated displays,projection systems, signs, etc.

In general, light having a first optical characteristic is emitted byone or more LEDs 1424 of the light source 1420, such light is directedinto one or more optical fibers 1413 through their input surfaces 1416by optical elements 1428. The light is directed by the optical fibers1413 through their output surfaces 1414 and illuminates the firstinterference reflector 1430. The first interference reflector 1430substantially transmits the light such that it illuminates emissivematerial 1440. The emissive material 1440 converts at least a portion ofthe light from the light source 1420 into light having a second opticalcharacteristic. Light emitted by the emissive material 1440 that isdirected toward the first interference reflector 1430 is substantiallyreflected by the first interference reflector 1430. If a LP interferencereflector (e.g., second interference reflector 150 of FIG. 2) isincluded in system 1400, then the light emitted by the emissive material1440 is substantially transmitted by the LP interference reflector. Anylight from the light source 1420 that illuminates the LP interferencereflector is substantially reflected back toward the emissive material1440 where it may then be converted into light having the second opticalcharacteristic. Light emitted by the emissive material 1440 and/ortransmitted by the optional LP interference reflector can then bedirected to a desired location using any suitable technique.

In some embodiments, the illumination systems 1300 of FIG. 14 and 1400of FIG. 15 may include a LP interference reflector and no SPinterference reflector. For example, FIG. 18 schematically illustratesan illumination system 1700 that includes an interference reflector 1750positioned to receive light from emissive material 1740. The system 1700also includes a light source 1720, and light guides 1712 opticallycoupled to the light source 1720. All of the design considerations andpossibilities in regard to the light guides 1312, the light source 1320,the emissive material 1340, and the optional second interferencereflector 1350 of the embodiment illustrated in FIG. 14 apply equally tothe light guides 1712, the light source 1720, the emissive material1740, and the interference reflector 1750 of the embodiment illustratedin FIG. 18. The system 1700 may include other features similar to thosedescribed in respect to illumination system 1300 of FIG. 14, e.g., oneor more optical elements, TIR promoting layers, etc.

In general, the light source 1720 emits light having a first opticalcharacteristic. Such light illuminates input surfaces 1316 of lightguides 1312 and is directed by the light guides 1712 through the outputsurfaces 1714 where at least a portion of such light illuminates theemissive material 1740. The emissive material 1740 emits light having asecond optical characteristic when illuminated with light having thefirst optical characteristic. At least a portion of the light emitted bythe emissive material 1740 illuminates the interference reflector 1740,which substantially transmits light having the second opticalcharacteristic and substantially reflects light having the first opticalcharacteristic. The substantially transmitted light is then directed toa desired location using any suitable technique.

The illumination systems of the present disclosure may be used in anysuitable manner for providing illumination. For example, some or all ofthe illumination systems described herein may be used to provideillumination for displays. FIG. 16 schematically illustrates a displayassembly 1500 that includes an illumination system 1510 opticallycoupled to a display device 1512. The illumination system 1510 mayinclude any illumination system described herein, e.g., illuminationsystem 10 of FIG. 1. The illumination system 1510 provides illuminationlight to the display device 1512. The display device 1512 may be anysuitable display device, e.g., LCD, electrochromatic or electrophoreticdevices, spatial light modulator(s), transmissive signs, etc.

For example, the display device 1512 may include one or more spatiallight modulators. In some embodiments, the one or more spatial lightmodulators may include an array of individually addressable controllableelements. Such spatial light modulators may include a suitable type ofcontrollable element. For example, the spatial light modulator mayinclude a variable-transmissivity type of display. In some embodiments,the spatial light modulator may include a liquid crystal display (LCD),which is an example of a transmission-type light modulator. In someembodiments, the spatial light modulator may include a deformable mirrordevice (DMD), which is an example of a reflection-type light modulator.

The display device 1512 may include any suitable optical and non-opticalelements for producing a display image, e.g., lenses, diffusers,polarizers, filters, beam splitters, brightness enhancement films, etc.The illumination system 1510 may be optically coupled to the displaydevice 1312 using any suitable technique known in the art.

All references and publications cited herein are expressly incorporatedherein by reference in their entirety into this disclosure. Illustrativeembodiments of this disclosure are discussed and reference has been madeto possible variations within the scope of this disclosure. These andother variations and modifications in the disclosure will be apparent tothose skilled in the art without departing from the scope of thedisclosure, and it should be understood that this disclosure is notlimited to the illustrative embodiments set forth herein. Accordingly,the disclosure is to be limited only by the claims provided below.

1. A display, comprising: an illumination system, wherein theillumination system comprises: a light source that emits lightcomprising a first optical characteristic; a plurality of light guidesoptically coupled to the light source, wherein each light guidecomprises an input surface and an output surface; emissive materialpositioned to receive light from at least one light guide of theplurality of light guides, wherein the emissive material emits lightcomprising a second optical characteristic when illuminated with lightcomprising the first optical characteristic; and a first interferencereflector positioned between the emissive material and the outputsurfaces of the plurality of light guides, wherein the firstinterference reflector substantially transmits light comprising thefirst optical characteristic and substantially reflects light comprisingthe second optical characteristic; and a spatial light modulatoroptically coupled to the illumination system, wherein the spatial lightmodulator comprises a plurality of controllable elements operable tomodulate at least a portion of light from the illumination system. 2.The display of claim 1, wherein the illumination system furthercomprises a second interference reflector positioned such that theemissive material is between the second interference reflector and thefirst interference reflector, wherein the second interference reflectorsubstantially transmits light comprising the second opticalcharacteristic and substantially reflects light comprising the firstoptical characteristic.
 3. The display of claim 1, wherein the pluralityof controllable elements of the spatial light modulator comprisesvariable-transmissivity display elements.
 4. The display of claim 3,wherein the variable-transmissivity display elements comprises liquidcrystal display elements.
 5. The system of claim 1, wherein the firstoptical characteristic comprises a first wavelength region and thesecond optical characteristic comprises a second wavelength regiondifferent than the first wavelength region.
 6. The system of claim 5,wherein the first wavelength region comprises UV light.
 7. The system ofclaim 5, wherein the first wavelength region comprises blue light. 8.The system of claim 1, wherein the emissive material is discontinuous.9. The system of claim 8, wherein the discontinuous emissive materialcomprises a plurality of dots of emissive material.
 10. The system ofclaim 9, wherein each dot of the plurality of dots of emissive materialhas an area of less than 10000 μm².
 11. The system of claim 9, whereinat least one dot of the plurality of dots of emissive material comprisesa first emissive material and at least one other dot of the plurality ofdots of emissive material comprises a second emissive material.
 12. Thesystem of claim 9, wherein the first optical characteristic comprises afirst wavelength region and the second optical characteristic comprisesa second wavelength region different than the first wavelength region,wherein at least a first dot emits light having a first peak wavelengthwithin the second wavelength region and at least a second dot emitslight having a second peak wavelength within the second wavelengthregion different than the first peak wavelength.
 13. The system of claim1, wherein the first optical characteristic comprises a first wavelengthregion and the second optical characteristic comprises a secondwavelength region different than the first wavelength region, whereinthe emissive material comprises a first index of refraction at the firstwavelength region.
 14. The system of claim 13, wherein the systemfurther comprises a TIR promoting layer in contact with the emissivematerial between the interference reflector and the emissive material,and further wherein the TIR promoting layer comprises a second index ofrefraction at the first wavelength region that is less than the firstindex of refraction.
 15. The system of claim 1, further comprising atleast one optical element positioned such that the interferencereflector is between the emissive material and the at least one opticalelement.
 16. The system of claim 15, wherein the at least one opticalelement comprises a reflective polarizer.
 17. A sign comprising anillumination system, wherein the illumination system comprises: a lightsource that emits light comprising a first optical characteristic; aplurality of light guides optically coupled to the light source, whereineach light guide comprises an input surface and an output surface;emissive material positioned to receive light from at least one lightguide of the plurality of light guides, wherein the emissive materialemits light comprising a second optical characteristic when illuminatedwith light comprising the first optical characteristic; and a firstinterference reflector positioned between the emissive material and theoutput surfaces of the plurality of light guides, wherein the firstinterference reflector substantially transmits light comprising thefirst optical characteristic and substantially reflects light comprisingthe second optical characteristic.